i think the best way to get in good shape with a bicycle is to ride all the damn time. i was never a serious cyclist, and then i started having a 2mile commute. rode all the time. then i lived somewhere where there was nothing to do but ride, so i rode all the time. then i had a 8mi commute, and friends on the other side of the city. now i'm fast and can ride all day - cause i do.

other things you can do if you want to increase your cycling abilities are interval training (sprint one minute, relax for five. repeat) and hill repeats (climb a hill. repeat).

good call on the cycling shorts. comfort on the bike is a big factor - makes you want to stay in the saddle more and more often.

get more friends into biking - your enthusiasm will get all synergetic, and be more than the sum of its parts.